Unity iOS plugin linker error

Hi, I encountered the following error while trying to build an iOS plugin I created for Unity. Can anyone please help me fix this error and successfully build this app?

Undefined symbols for architecture armv7:

  "_Foo", referenced from:

      _Bar_Foo_m2346119406 in Bulk_Assembly-CSharp_0.o

    (maybe you meant: _Bar_Foo_m2346119406)

ld: symbol(s) not found for architecture armv7

clang: error: linker command failed with exit code 1 (use -v to see invocation)

Here's my Unity C# code:

Code (CSharp):

1. public class Bar : MonoBehaviour {
2. ...
3.     [DllImport("__Internal")]
4.     private static extern void Foo(string URL, string Quote);
5. ...
6. }

Here's my Objective-C header:

Code (CSharp):

1.  
2. //
3. //  FooUtil.h
4. //
5. //
6.  
7. #ifndef FooUtil_h
8. #define FooUtil_h
9.  
10. #import <Foundation/Foundation.h>
11. #import <UIKit/UIKit.h>
12.  
13. @interface FooUtil : NSObject
14.  
15. + (void)Foo:(const char*)URL :(const char*)Quote;
16.  
17. @end
18.  
19. #endif /* FooUtil_h */

Here's my class file code in Objective-C:

Code (CSharp):

1. //
2. //  FooUtil.m
3. //
4.  
5. #import "FooUtil.h"
6. #import <Foundation/Foundation.h>
7.  
8. @interface FooUtil ()
9. @end
10.  
11. @implementation FooUtil
12. + (void)Foo:(const char*)URL :(const char*)Quote {
13. }
14.  
15. @end
16.  

I've never actually written Objective-C code so I have no idea what I'm doing.

 

Okay I successfully built the plugin by removing the header file, and changing my Objective-C code to the following:

Code (CSharp):

1.  
2. //
3. //  FooUtil.m
4. //
5. #import <Foundation/Foundation.h>
6.  
7. void Foo(const char* URL, const char* Quote) {
8. }